struct MFR { 
....   
};   

struct MFRL {
....
};

struct directory {
  char ftype;
  ...
}; 

union block {
  struct directory d;
  struct MFR  m;
  struct MFRL l;
  char s[64];
};

union block *disk[256];

int main ()
{
     union block u;
     u.d.ftype = 'D';
     
     disk[1]=malloc(64);
     disk[1]->d.ftype='D';
     disk[2]=malloc(64);
     strcpy(disk[2]->s,"The quick brown fox");
     return 0;
}